Two probe standing wave null detector



Jan. 24, 1967 c. P. TRESSELT 3,309,715

TWO PROBE STANDING WAVE NULL DETECTOR Filed Jan. 24, 1963 O. w SHOBR 25SOURCE JFCD 22 2/ MOVABLE F/XED PROBE PROBE VOLTAGE (E-F/EL D) STAND/NOWAVE DIFFERENCE ARM 3 SYMMETR/C ARMS FFQTI 50 Q: g 2 Q q 38 d it & ERRORE 39 S/GNAL DISPLACEMENVT 0F PROBE 27 23 1 2/0 240 FREQUENCY MOVA BLEN/XED COUNTER PROBE; RROBE K VOL TAGE CONTROLL- ED LOCAL OSC/LL.

47 VOLTAGE CONTROLLED OSC/LL.

COMP. INVENTOR.

Q CARL F. TRESSELT United States Patent Office Patented Jan. 24, 1967 3300 715 TWO PROBE STANDIDIG vvAvE NULL DETECTOR Carl P. Tresselt,Detroit, Mich., assignor to The Bendix Corporation, Southfield, Mich., acorporation of Delaware Filed Jan. 24, 1963, Ser. No. 253,564 8 Claims.(Cl. 32458.5)

This invention pertains to a two probe device for detecting nulls of astanding wave in a transmission line utilizing a probe which has a fixedentrance into the transmission line and is capable of sensing themagnitude and phase of the standing wave in the transmission line at thepoint of entrance and a probe which has a movable entrance into thetransmission line and is also capable of sensing the magnitude and phaseof the standing wave at its point of entrance.

Two probe detectors are known to the art, for example, US. Patent No.2,967,996, Magnetron Tester, by Henry K. Bradford, issued Jan. 10, 1961,illustrates a typical system having two probes in fixed relation to eachother and which are movable relative the transmission line. This typesystem has proven disadvantageous because the null would be located bycentering it between the two probes. Each probe would causediscontinuities at the point of measurement which set up standing wavesof their own and hence interfere with accurate measurements. In thisinvention only one probe is used at the point of measurement and thisprobe locates the null by being positioned exactly at the null wherethere is zero field so that any discontinuity effect is negligible.Also, in the prior systems the probes themselves and all the elements ineach probe circuit had to be very carefully matched in order foraccurate measurements whereas in applicants system such matchingrequirements are not necessary. The preferred embodiment of thisinvention is adapted to be used with the invention in a copendingapplication entitled Measuring Gauge, Serial No. 216,592, filed August13, 1962, by Carroll F. Augustine and myself. In the preferredembodiment the probe is moved and then the null is moved until it isunder the probe. However, the principles are similar whether the probeis moved to find the null or the null is moved to find the probe.

In this invention one probe is fixed at a point in a slottedtransmission line approximately one quarter of the standing wave lengthfrom the general area where the standing wave null will detect themovable proble position. The movable probe is moved in this area by anobject the displacement of which is to be measured. The fixed probe ison the side of the movable probe which is away from the shorted end ofthe transmission line so that the discontinuities cause by the fixedprobe will have a minimum effect on the standing wave. The frequency inthe transmission line is varied until the null comes under the new probeposition and by measuring the frequency, a very accurate determinationof probe movement may be obtained. This measuring concept is covered indetail in the above cited copending application.

In this invention means for measuring the phase difference between thefixed and movable probes are provided. In a first embodiment the probeoutputs are connected to symmetric arms of a magic tee while the sum armand difference arm of the magic tee are connected to oppositelypolarized diodes. The output of the diodes is measured and when it iszero, the moving probe is at a null, and when it has a plus sign it ison one side of a null and a minus sign it is on the other side of anull. A servo system may then be employed to change the frequency in thetransmission line until a null is at the movable probe whereby the exactdisplacement of the movable probe may be determined.

A further embodiment of this invention is comparing the phase betweenthe movable and fixed probe. In this embodiment a movable and fixedprobe are connected to mixers which are driven by a local oscillator. Bycomparing the phase from the mixers it can be determined when themovable probe is at a null. This superheterodyne embodiment has theadvantage of lower noise and the ability of amplify at an intermediatefrequency level.

In the last embodiment, a servo system also may be employed to changethe frequency of the signal in the transmission line until a nodeappearsat movable probe to exactly measure the displacement of themovable probe.

It is therefore an object of this invention to provide a two probe nulldetector system with discontinuities set up by the probes having aminimum effect on the null detection construction. phase comparisonsystems for said probes of inexpensive and reliable operation.

These and other objects will become more apparent when preferredembodiments are considered in connection with the drawings in which:

FIGURE 1 is a schematic diagrammatic drawing of a first embodiment ofthis invention utilizing a magic tee as the phase comparator;

FIGURE 2 is a graph showing the error signal versus the displacement ofthe movable probe; and

FIGURE 3 is a schematic diagrammatic view of a second embodiment of thisinvention utilizing a superheterodyne circuit for comparing the phase inthe movable and fixed probe.

In FIGURE 1 is shown a transverse electromagnetic (T.E.M.) slottedtransmission line 21 which has a constant wave signal source 22 at oneend thereof and is shorted at end 23. A standing wave 24 is formed inthe transmission line 21 and it is desirable to place the node 25 of thestanding wave exactly over a predetermined point such as the position ofa movable probe 27. This can be done by varying the frequency of source22 until the node 25 is over probe 27. In this application it isdesirable to know exactly how far the probe 27 is from the shorted end23 and this can be determined by knowing the frequency necessary toplace node 25 over probe 27.

The means for determining when node 25 is precisely, over, probe 27 willnow be described. A second probe 28 is fixed in relation to transmissionline 21 and is placed on the side of probe 27 away from shorted end 23so that any discontinuities caused by probe 28 will not atfect thestanding wave 24 at the point of measurement. The fixed probe 28 will besomewhere near the maximum voltage of the standing wave, but need not beat the maximum voltage, since the movement of movable probe 27 will beWithin small limits compared with the length of the standing wave 24.

The output of the movable probe 27 is connected to one of the symmetricarms 30 of a magic tee and the output of the fixed probe 28 is connectedto the other symmetric arm 31 of the magic tee. It is a property of themagic tee to add the signals from arms 30 and 31 in the sum arm 32 andsubtract the signals in the difference arm 33. This is explained inElectronic and Radio Engineering by Terman, fourth edition, published byMcGraw-Hill on pp. 156-157. The time phase of the signal on one side ofthe null in the standing wave is almost exactly out of phase with thesignal on the opposite side of the null, due to the essentially infinitemagnitude of the standing wave ratio. Hence the signals A further objectis to provide processed by the magic tee are either in or directly outof phase and thus add algebraically; the magnitude of the radiofrequency signal at one arm of the magic tee output will be larger thanthat of the other, the order depending upon which side of the null themovable probe is 5 located.

The sum arm 32 is connected to a detector 34 having one polarity and thedifference arm 33 is connected to a second detector 35 having theopposite polarity. The outputs of detectors 34 and 35 are connectedacross a resistance 36 and a pointer 37 which is adjusted to give a zeroreading across terminals 38 and 39 when probe 27 is at null 25. Inpractice a servo system would be connected across terminals 38 and 39 tovary the frequency of standing wave 24 until the node 25 was inalignment with movable probe 27. When aligned, the discontinuitiescreated by probe 27 have a minimum effect on the standing wave 24,improving accuracy.

In FIGURE 2 is shown a graph of the error signal which appears acrossterminals 38 and 39 versus the displacement of movable pro-be 27. It canbe seen that as null moves to one side of probe 27, a minus voltage ispicked up by the probe and as it moves to the other side a positivevoltage is picked up by the probe.

A second embodiment shown in FIGURE 3 for measuring the phase differencebetween a fixed probe 28a and a movable probe 27a. In this embodimentthe outputs of probe 27a and 28a, respectively, are connected to mixers41 and 42 which are driven by voltage controlled local oscillator 43.The difference frequencies or intermediate frequencies from the mixer 42is amplified by a limiter intermediate frequency amplifier 44 whichkeeps its output at constant amplitude and feeds this to phasecomparator 46. The output of mixer 41 is connected to a linearintermediate frequency amplifier 45 and its output is also connected tophase comparator 46 which compares the phase of the outputs of the twoamplifiers 44 and 45 and drives a voltage controlled oscillator 47 whichestablishes the signal in slotted line 21a. Voltage controlledoscillator 47 may be of the type having a voltage variable capacitor(varactor) as part of a resonant circuit of a conventional oscillator.

If probe 27a is to the right of the null a positive signal is produced,and to the left of null 25 a negative signal is produced in a manneranalogous to the magic tee-diode detector arrangement of the firstembodiment, which is simulated at intermediate frequencies by the phasecomparator. These signals are sent to voltage controlled oscillator 47thereby controlling its output frequency in line 21a so that the null25a corresponds with the probe position 27a, closing the servo loop.

The use of the superheterodyne circuit in FIGURE 2 not only reducesnoise but also permits intermediate frequency amplification of thesignal to phase comparator 46, improving the sensitivity of the system.

In order to keep the intermediate frequency from mixers 41 and 42 in theband pass of amplifiers 44 and 45 and phase comparator 46, adiscriminator 50 receives the frequency from amplifier 44 and changesthis to a voltage corresponding to the frequency thus tuning voltagecontrolled oscillator 43 so that the intermediate frequency ismaintained in the band pass regions.

Frequency readout 51 indicates the frequency change so that the movementof probe 27a is accurately known.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

Having thus described my invention, I claim:

1. Voltage measuring device for measuring standing waves in Waveguidemeans comprising waveguide means,

frequency generating means for effecting a standing wave in saidwaveguide means,

first probe means for measuring a reference voltage cor responding to aportion of the standing wave voltage in said waveguide means,

second probe means moavble relative to said first probe means andmovable along the waveguide means to measure the standing wave voltagein said waveguide means,

comparing means,

said first and second probe means being connected to said comparingmeans to compare the reference voltage and the standing wave voltagethereby developing a sign-a1 corresponding to the relative osition ofsaid second, probe means and said standing Wave,

said first probe means being positioned relative to said waveguide meansto measure a portion of said standing wave in said waveguide means asthe reference voltage,

said second probe means being connected to and moved by an object thedisplacement of which is to be measured,

voltage controlled oscillator means to change the frequency of thesignal in said waveguide means,

said oscillator means being connected between the output of saidcomparing means and said frequency generating means to change thefrequency of the signal in said waveguide means until the comparingmeans output sends a zero signal to said oscillator means,

means to count the frequency change.

2. Voltage measuring device for measuring standing waves in Waveguidemeans comprising waveguide means,

frequency generating means for effecting a standing wave in saidwaveguide means,

first probe means for measuring a reference voltage corresponding to aportion of the standing wave voltage in said waveguide means,

second probe means movable relative to said first probe means andmovable along the Waveguide means to measure the standing wave voltagein said waveguide means,

comparing means,

said first and second probe means being connected to said comparingmeans to compare the reference voltage and the standing wave voltagethereby developing a signal corresponding to the relative position ofsaid second probe means and said standing wave,

a magic tee having two symmetric arms, a sum arm and a difference arm,

said first probe means being connected to one of said symmetric arms andsaid second probe means being connected to the other of said symmetricarms,

a rectifier being connected to the sum arm for passing current in onedirection from said magic tee and a rectifier being connected to saiddifference arm for passing current in the other direction from saidmagic tee,

said rectifiers being connected to resistance means across which theoutput signal is developed.

3. Voltage measuring device for measuring standing waves in waveguidemeans comprising waveguide means,

frequency generating means for effecting a standing wave in saidwaveguide means,

first probe means for measuring a reference voltage corresponding to aportion of the standing wave voltage in said waveguide means,

second probe means movable relative to said first probe means andmovable along the waveguide means to measure the standing wave voltagein said waveguide means,

comparing means,

said first and second probe means being connected to said comparingmeans to compare the reference voltage and the standing wave voltagethereby developing a signal corresponding to the relative position ofsaid second probe means and said standing wave,

said first probe means is positioned relative to said means to measurethe standing wave voltage in said conductor means,

means for providing movement of said second probe means along saidconductor means in response to waveguide means to measure a portion ofsaid stand 5 the object displacement between a reference position ingwave in said waveguide means as the reference and a position to bemeasured, voltage, said first and second probe means being sensitive tothe said second probe means is connected to and moved amplitude of theelectromagnetic standing Wave in by an object the displacement of whichis to be said conductor means, measured, 10 comparing means, voltagecontrolled oscillator means to change the fresaid first and second probemeans being connected to quency of the signal in said waveguide means,said comparing means to compare the reference voltsaid oscillator meansbeing connected between the outage and the standing wave voltage therebydeveloping put of said comparing means and said frequency gena positionsignal corresponding to the relative posicrating means to change thefrequency of the signal tion of said second Probe means and SaidStanding in said waveguide means until the comparing means wave, outputsends a zero signal to said oscillator means, means for changing thefrequency of the signal in said means to count the frequency change,generating means after the second probe means has two mixers, been movedso that a null can be obtained at said a local oscillator driving saidmixers, second probe means, said first probe means being connected to afirst of said means for indicating the frequency Change 50 that themixers and said second probe means being connected disp of Said SecondProbe means can be to the second of said mixers, obtained. a phasecomparator, 7. The apparatus of claim 6 with said mixers being connectedto said phase comparator. Said means for Changing the frequency beingresponsive 4. The device of claim 3 where said local oscillator is toSaid P i i-OII gn voltage controlled, 8. A method comprising the stepsof a discriminator being connected between said first establishing anelectromagnetic standing Wave,

mixer to change frequency changes from said first measuring On portion fthe Standing Wave, mixer to voltage changes to said controlled local 05-mwsllring a sacond Portton of th? Standing Wave at a cillator so thatsaid last named oscillator will genfirst Position spflcfid from Saidfifstmeasufemfint, erate a frequency which when subtracted from themoving Said SeCOBd mfiasurement Point t0 Sticond probe frequencies willresult in a substantially con- Position on electromagnetic Standing Wave00mita t inte diat frequency, sponding to a displacement to be measured,

5, Th device ofl l i 3 havi changing the frequency of the standing waveso that a a limiter intermediate frequency amplifier between said null0f the Standing Wave can be Obtained at the fi t mixer and phase tosecond measurement position of the standing Wave,

a linear intermediate frequency amplifier being between measuring thefrequency Change required to Obtain id second mixer d id phasecomparator null point at the second position in order to deter- 6,Apparatus comprising 40 mine the displacement of the second measurement."eneratin means for ener=atin a variable hi h frequgncytelectromagneciicsignal? D References Cited by the Examiner conductor means for receivingsaid high frequency elec- UNITED STATES ATE TS tromagnetic signal at oneend thereof, 2 01 27 7/1952 Miner 3 X means for terminating the otherend of said conductor 2 758 663 3 Snavely,

means to produce an electromagnetic standing wave 2 9 7 99 1 19 1 B dfod 24 g1 X therem, 3,122,665 2/1964 Bailey 324- 68 first probe means formeasuring a reference voltage corresponding to a portion of the standingwave voltage in said conductor means,

second probe means movable along the conductor WALTER L. CARLSON,Primary Examiner. 0 R. V. ROLINEC, A. E. RICHMOND, P. F. WILLE,

Assistant Examiners.

1. VOLTAGE MEASURING DEVICE FOR MEASURING STANDING WAVES IN WAVEGUIDEMEANS COMPRISING WAVEGUIDE MEANS, FREQUENCY GENERATING MEANS FOREFFECTING A STANDING WAVE IN SAID WAVEGUIDE MEANS, FIRST PROBE MEANS FORMEASURING A REFERENCE VOLTAGE CORRESPONDING TO A PORTION OF THE STANDINGWAVE VOLTAGE IN SAID WAVEGUIDE MEANS, SECOND PROBE MEANS MOVABLERELATIVE TO SAID FIRST PROBE MEANS AND MOVABLE ALONG THE WAVEGUIDE MEANSTO MEASURE THE STANDING WAVE VOLTAGE IN SAID WAVEGUIDE MEANS, COMPARINGMEANS, SAID FIRST AND SECOND PROBE MEANS BEING CONNECTED TO SAIDCOMPARING MEANS TO COMPARE THE REFERENCE VOLTAGE AND THE STANDING WAVEVOLTAGE THEREBY DEVELOPING A SIGNAL CORRESPONDING TO THE RELATIVEPOSITION OF SAID SECOND PROBE MEANS AND SAID STANDING WAVE, SAID FIRSTPROBE MEANS BEING POSITIONED RELATIVE TO SAID WAVEGUIDE MEANS TO MEASUREA PORTION OF SAID STANDING WAVE IN SAID WAVEGUIDE MEANS AS THE REFERENCEVOLTAGE, SAID SECOND PROBE MEANS BEING CONNECTED TO AND MOVED BY ANOBJECT THE DISPLACEMENT OF WHICH IS TO BE MEASURED, VOLTAGE CONTROLLEDOSCILLATOR MEANS TO CHANGE THE FREQUENCY OF THE SIGNAL IN SAID WAVEGUIDEMEANS, SAID OSCILLATOR MEANS BEING CONNECTED BETWEEN THE OUTPUT OF SAIDCOMPARING MEANS AND SAID FREQUENCY GENERATING MEANS TO CHANGE THEFREQUENCY OF THE SIGNAL IN SAID WAVEGUIDE MEANS UNTIL THE COMPARINGMEANS OUTPUT SENDS A ZERO SIGNAL TO SAID OSCILLATOR MEANS, MEANS TOCOUNT THE FREQUENCY CHANGE.